Petrol engines equipped with direct fuel injection have recently been introduced onto the market, these being engines in which the petrol is injected directly into the cylinders by suitable injectors, each of which is arranged in the crown of a respective cylinder and is current-driven by a central control unit. Usually, the central control unit is capable of causing a time-variable current wave to flow through an injector drive circuit, said wave being intended to generate a force of an electromagnetic nature in order to displace the injector plunger from a closed position to an open position against the action of a spring that tends to hold the plunger in the closed position.
Usually, a central control unit also implements diagnostic functions designed to verify the correct operation of the various engine components, in order either to notify the driver of the necessity for maintenance or to use a special control strategy that takes account of any failures or malfunctions. The various diagnostic functions provided in a known control unit usually include verification that movement of the plunger of each injector has occurred; in other words, for each injector a check is made of whether the injector plunger has actually opened or closed after each injection command. Currently, verification that movement of the plunger for each injector has occurred is carried out by means of a software verification strategy, which is deductive in nature and is carried out on the engine system after a relatively long period of time; however, this verification strategy is slow, somewhat inaccurate and entails the use of considerable computing resources.
In known central control units, it is also possible to measure the electrical resistance of the drive circuit for each injector in order to verify whether the drive circuit is open, short-circuited or whether the drive circuit has a terminal shorted towards the positive or negative pole of the battery of the vehicle; however, the plunger of an injector could be jammed even if the associated drive circuit exhibits no obvious defects and this verification method thus does not make it possible to identify all possible injector malfunctions. Moreover, the ongoing increases in fuel injection pressures are accompanied by a consequent increase in the control currents and thus a decrease in the electrical resistance of the injector drive circuits; it is thus increasingly difficult and complicated to measure the resistance of the drive circuit with an accuracy sufficient to differentiate a short circuit condition from an acceptable operating condition.
Finally, a proposal has been made to use appropriate dedicated sensors (accelerometric, pressure or positional) attached to the injectors in order to monitor the correct movement of the plunger; however, this solution is extremely expensive owing to the costs of purchasing and installing the sensors.